Automatically change the labelling style depending on the visibility of each label on a layer?

Question

I would like to show as many street name/label as possible on the print composer map, without overlapping, automatically change each label’s style, if it is necessary, depending on what is possible to display.

The default label is curved, 8p (Liberation Sans), but if the QGIS can't show a label (because of any reason), then the style of label of that street should change to a narrow font (Liberation Sans Narrow). If it is not possible at all to show a label in curve, then it should change to parallel label.

Summarize (for each label on the layer):

• If it is possible to show: Curved Label, normal font
• Else if it is possible to show: Curved Label, narrow font
• Else if it is possible to show: Parallel Label, narrow font
• Else: Parallel Label (just the first 3 character and a ‘.’ For example: “Main street” -> “Mai.”)

How is it possible in QGIS?

Answer 1

Unfortunately I don't think it is possible to set visibility conditions this way.

Your only option is a binary choice: show colliding labels, or hide colliding labels:

Ideally, the expression builder would have a variable or function to test whether a given label will be displayed or hidden. If QGIS had such a variable, you could use data-defined settings like if( labels_collide = 'true', ... , ...).

It might be possible to create a custom function. Once created, you can load a custom function into QGIS and it will be available in the Expression Builder alongside the standard functions. I don't know how to do this, but I think it's possible. If you want to learn more, here are a few links that to get you started:

• How do I declare a QGIS custom expression function and use it in a single python script?

• PyQGIS Developer Cookbook